How to adapt my circuit in the PCB??

[samsoun]

Hello,
I am working in a Printed circuit board, when I calculate the width traces I find it about 2 mm to be adapted at 50ohm. But 2 mm is too long, so, If I put the width traces 1mm, how can I adapt my circuit at 50ohm???
Should I add an impedance matching circuit ??
Thanks )

 

[FvM]

You rarely need many 50 ohm traces in a circuit, so what's the problem with 2 mm trace width? For smaller form factor, use thin PCB substrate or multilayer, the trace widths will be respectively downscaled.

 

[samsoun]

many thanks, i limited by the size of the PCB and the components. I dont have a choice :/

 

[Dan Mills]

A buried ground plane with a thin substrate between it and the layer carrying your 50 ohm traces will make the traces thinner for a given impedance, run the numbers.

You probably need the plane anyway, so may as well tweak the layer stack to make things work.

Can you simply get everything within ~1/10th wavelength at your operating frequency and thus make track behaviour approximate lumped constant?

73 Dan.

 

[samsoun]

Unfortunately, I have only 1 Double layer PCB, and I have to deal with it. And my operating frequency is 868MHZ (ISM band).

 

[FvM]

If the PCB is really small, a substrate height of 0.5 or 0.8 mm seems appropriate. It's better for a RF circuit anyway due to reduced via inductance.

Another option to reduce trace width for 50 ohm nets is to design it as coplanar waveguide with or without ground instead of microstrip.

 

[volker@muehlhaus]

And my operating frequency is 868MHZ (ISM band).

What is the length of the line?